1. Field of the Invention
The present invention relates to an optical disk or a disk-shaped recording medium in which reflected light from it is utilized in order to reproduce signal information recorded thereon.
2. Description of the Related Art
Pits corresponding to signal information to be stored are formed in the optical disk such a compact disk (CD) or a video disk. In an optical disk player reproducing the signal from the optical disk, a laser beam is focused as a spot on pits or non-pit portions and then a change in the intensity of the reflected light therefrom is detected by its photodetector to reproduce the recorded signals. The optical detection system in this optical disk player has a limitation on the cutoff spatial frequency 2 NA/.lambda. (wherein NA represents the numerical aperture of the objective lens; and .lambda. represents the wavelength of the laser beam). To improve the spatial resolution to thereby increase the surface density of the recording film, it is essential to increase the numerical aperture of the objective lens and shorten the wavelength of the laser beam.
As an example of an optical disk player designed to improve the spatial resolution, there has been proposed an apparatus which employs apodization or super resolution (as disclosed in Japanese Patent Application koukai No.2-12625/1990). The optical irradiation system in this optical disk player decreases the light intensity near the center of the entrance pupil of a read laser beam by using a light-shielding plate before forming the spot of a diffraction pattern on the surface of an optical disk. This optical irradiation system utilizes a main lobe of a small diameter size in the light intensity distribution of the apodization spot formed on the optical disk.
In case such a light intensity distribution of the spot is taken into consideration, there may be offered an optical disk comprising a reflection layer made of a phase changing material such as SbSe with a temperature-on-reflectance dependency such that the reflectance increases in accordance with an increase of temperature by the irradiation of the laser beam.
Such an optical disk has a characteristic of repetition of reading data which is restricted in the repeating numbers of reading, because the reflection layer of phase changing material changes in its molecular structure by the irradiation of the laser beam. Generally, it is desired optical materials having a high response speed to the irradiation of the laser beam.